A baculovirus expression system enables high-level expression of a gene of interest by causing recombination of the gene in Sf9 cells using a promoter of a polyhedrin gene of baculovirus. A polyhedrin is expressed in the nucleus of Sf9 cell at a high level as an occlusion body which is a form to be used when viruses become latent within the cell. The baculovirus expression system where a recombinant protein is introduced into a polyhedrin gene and the expressed proteins are purified, has many advantages over that of Escherichia coli, such that the expressed proteins are hard to agglutinate and the expressed proteins undergo a posttranscriptional modification which is necessary for protein functions, such as addition of sugar chains and coordination of metal ion.
Baculovirus has another life cycle. In order to proliferate and infect, baculovirus becomes a budded virus (Budded virus: this is also referred to as budded baculovirus in this specification), rupturing Sf9 cell membrane and being released outside the cell. Bouvier et al have reported that at this time a receptor of seven-transmembrane type recombined into the above polyhedrin protein is expressed on the cell membrane and recovered from the envelope of the budded baculovirus (Loisel T P, Ansanay H, St-Onge S, Gay B, Boulanger P, Strosberg A D, Marullo S, Bouvier M., Nat Biotechnol. 1997 November; 15(12): 1300–4., Recovery of homogeneous and functional beta 2-adrenergic receptors from extracellular baculovirus particles; and International Publication WO98/46777). It has also been reported that, whereas most receptors of seven-transmembrane type expressed in a host cell have a sugar chain structure which is not functional, only functional receptors are recovered from the viral envelope. However, Bouvier et al have not mentioned membrane proteins other than receptor proteins.